Detection of extended-spectrum beta-lactamases in klebsiellae with the Oxoid combination disk method

Abstract

The Oxoid combination disk method for detecting extended-spectrum beta-lactamases (ESBLs) depends on comparing the inhibition zones of cefpodoxime (10-microg) and cefpodoxime-plus-clavulanate (10- plus 1-microg) disks. The presence of clavulanate enlarged the zones for all of 180 ESBL-producing klebsiellae by >/=5 mm, whereas zones for cefpodoxime-susceptible isolates and cefpodoxime-resistant isolates with AmpC and K1 beta-lactamases were enlarged by </=1 mm. Good discrimination was achieved with either the NCCLS or the British disk method.

FIG. 1

Zone distributions by NCCLS methodology. (A) Distribution of inhibition zone differences (CD01 zone minus CPD zone) for ESBL producers (solid bars) and non-ESBL producers (open bars). (B and C) Zone diameters of CPD (10-μg) disks and CAZ (30-μg) disks, respectively, for ESBL producers (solid bars) and non-ESBL producers (open bars). Zone diameters indicating resistance, intermediate resistance, and possible ESBL positivity according to the NCCLS are situated to the left of the dashed, dotted-and-dashed, and solid vertical lines, respectively.

FIG. 2

Zone distributions by BSAC methodology. (A) Distribution of inhibition zone differences (CD01 zone minus CPD zone) for ESBL producers (solid bars) and non-ESBL producers (open bars). (B and C) Zone diameters of CPD (10-μg) disks and CAZ (30-μg) disks, respectively, for ESBL producers (solid bars) and non-ESBL producers (open bars). Zone diameters indicating resistance according to the BSAC are situated to the left of the solid vertical line.